Metal complex salt crystals exhibiting a specific metachromatism have heretofore been used as thermochromic materials. These metal complex salt crystals, however, have thermochromic temperature ranges of substantially from 50.degree. C. to several hundred degrees centigrade (.degree.C.). Most conventional thermochromic materials have thermochromic temperature ranges exceeding 100.degree. C. Furthermore, there are no thermochromic materials exhibiting a metachromatism within temperature ranges which are used in everyday life. Thus, the conventional thermochromic materials can be used only in limited applications.
When using conventional thermochromic materials, it has not been possible to cause them to produce the desired color at the desired temperature. This is true because there are only a few substances which exhibit metachromatism at temperatures lower than 100.degree. C. For example, a compound of Ag HgI turns from yellow to orange at 50.degree. C., and a compound of Cu HgI from red to brown at 70.degree. C. Therefore, it is not possible to select the type of color, and, furthermore, any alternation in color is not distinguishably clear.
Moreover, the metal complex salt crystals do not have any light transparency and, therefore, it is not possible to alternately cover and reveal the background by a means such as temperature changes. Many of these complex salts contain heavy metals, and in particular, the substances exhibiting a metachromatism at temperatures lower than 100.degree. C. contain mercury as described hereinbefore. These substances, therefore, require sufficient attention in handling due to possible pollution related problems. Thus, it is not desirable to use these substances in large amounts.
Other thermochromic materials include liquid crystals. These liquid crystals have thermochromic temperature ranges of from about -10.degree. C. to about 200.degree. C., and there are only one or two liquid crystals which exhibit a metachromatism at temperatures lower than 0.degree. C. As is the case with the above-described metal complex salt crystals, therefore, it is not possible to obtain the desired color at the desired temperature using such liquid crystals. It is only possible to obtain the color that is inherent to the liquid crystal. Furthermore, the crystals are undesirable because the effect inevitably degrades when they are brought into contact with other substances because they are chemically very sensitive. Thus, it is not advantageous to use such liquid crystals as thermochromic materials. Disadvantages of these materials are clear considering that: (1) there can be obtained only materials of dark color tone since cholesteric liquid crystals require a black background; and (2) such compounds are very expensive.
A considerable number of applications of metachromatism, i.e., the phenomenon of alternation in color due to changes in temperature are known. However, no suitable thermochromatic materials have been discovered. Therefore, for sometime there has been a need to develop materials exhibiting excellent metachromatism.
The applicant has already proposed reversible thermochromic materials which exhibit a metachromatism within a temperature range of from minus several ten of degrees centigrade to about 100.degree. C. These materials make it possible to select the desired color (see U.S. Pat. No. 4,028,118, corresponding to British Pat. No. 1,405,701, French Patent 73 19876, and German Pat. (OLS) No. 2,327,723). These thermochromic materials provide deeper color at temperatures below the temperatures at which they undergo the metachromatism (i.e., below the temperature of coloration/decoloration) than the color at temperatures above the temperatures of coloration/decoloration thereof. (These thermochromic materials described in U.S. Pat. No. 4,028,118 are referred to "a lower temperature side coloring type" thermochromic material.)
The present invention is intended to provide reversible thermochromic materials which provide deeper color at temperatures above the temperatures of coloration/decoloration than the color at temperatures below the temperature of coloration/decoloration. (The thermochromic materials mainly intended by the present invention are referred to "a higher temperature side coloring type" thermochromic material.) However, the materials of the invention are equal to the above thermochromic materials in that they exhibit metachromatism within the abovedescribed temperature range and provide the desired color. The higher temperature side coloring type reversible thermochromic materials of the invention make up for the deficiency of the above-described lower temperature side coloring type thermochromic materials. This increases the value and the possible uses of the materials of the invention.
Investigations have been carried out on the phenomenon of coloration/decoloration resulting from the reaction of electron-donating, chromatic, colorless organic compounds with acidic phosphoric acid ester compounds. As a result of these investigations, it has been found that the metachromatism proceeds thermally reversibly. Based on the discovery that such reversible metachromatism occurs due to a relatively large dependency on the temperature of acid-dissociation of acidic phosphoric acid ester compound, further study was continued and, as a result, it has been found that the coloring type and the temperature of coloration/decoloration at which the reversible thermochromic material undergoes a metachromatism are determined by physical properties of acidic phosphoric acid ester compound, such as molecular weight, melting point, solidifying point, and oxidiation.
Furthermore, it has been found that the use of one or more compounds selected from the group consisting of alcohols, esters, ketones, ethers, acid amides, and carboxylic acids in combination permits production of thermochromic materials which are more useful and practical. More specifically, the use of one or more compounds selected from the group consisting of alcohols, esters, ketones, ethers, acid amides, and carboxylic acids in combination causes coloration/decoloration to occur over wider temperature ranges and more delicately. This alternation in color proceeds sharply and sensitively, and reversibly from colorless to transparent color, from Color (I) to Color (II), or from color to transparent colorless. Furthermore, the reversible thermochromic material of the invention has the ability to cover or reveal the background.